The raw gas can be produced by combustion of catalytic conversion of hydrogen-sulfide-containing waste gases that accumulate, for example, during the purification of coke-oven gases. The raw gas mostly has an SO2 content between 3 and 12% (v/v). The gas is usually cooled in a waste-heat boiler and optionally diluted with air to establish a superstoichiometric O2/SO2 ratio. The exothermic reaction of SO2 to give SO3 takes place over catalysts, e.g. V2O5 catalysts at temperatures between 400 and 650° C. The SO3 hereby formed is cooled and absorbed in concentrated sulfuric acid in an absorber. In the known method, which is also designated as a contact method in the literature, SO2 and SO3 in the form of a sulfuric-acid mist occur as substantial emissions. The SO2 emissions result from an incomplete conversion of SO2 to SO3.
Unconverted SO2 is merely absorbed to a very small extent in the absorber. The SO2 emission can be up to 3000 mg/Nm3 if suitable measures are not taken to reduce emissions.
Numerous measures for reducing SO2 emissions are known. A high O2/SO2 ratio can thermodynamically promote the conversion of SO2 at a generally lower temperature level. The use of a cesium-doped catalyst can improve the conversion and achieve lower SO2 emissions. Finally, in order to improve the degree of conversion, the number of catalyst stages can be increased or an intermediate absorber can be provided before the last catalyst stage. However, in some cases, the operating costs of a sulfuric-acid recovery plant are increased significantly by the measures described. Furthermore, the measures are complex in terms of process technology and plant technology and are only suitable for retrofitting existing sulfuric-acid recovery plants to a limited extent. An overview of the known measures for reducing SO2 emissions in a sulfuric-acid recovery plant is given in the publication of H. Wiesenberger “State of the art in sulfuric acid production,” Monographs, Vol. 137 (2001), pages 7 to 23, ISBN 3-8557-583-1.
Gypsum suspension scrubbers are used to separate SO2 in power plants, waste incineration plants and industrial installations where a high degree of separation is achieved. In coking plants or petrochemical plants having a downstream sulfuric-acid recovery plant, a gypsum-suspension scrubber is not generally available. The SO2 emission cannot be introduced into the H2S scrubbers of a purification plant for coke-oven gas or corresponding gas scrubbers in a petrochemical plant since the salts produced interfere with the downstream stages of a conventional coke-gas purification system or corresponding equipment in a petrochemical plant.
German patent 26 58 208 discloses a method of eliminating sulfur oxides from a waste gas produced during the combustion of hydrocarbons in which sulfur oxides contained in the waste gas are reduced to hydrogen sulfide in a catalytic conversion zone. The hydrogen sulfide is extracted from the waste gas stream so that the purified waste gas stream only contains small quantities of sulfur.
It is known from U.S. Pat. No. 2,992,884 to reduce residual gases from sulfuric acid production with hydrogen.
In a method known from German patent document 21 66 916 [GB 1,344,471], waste gases are purified by catalytic hydrogenation of sulfur, the sulfur dioxide contained in the waste gases being converted into H2S using a stoichiometric quantity of hydrogen and into sulfur following absorption in an aqueous alkaline solution.
U.S. Pat. No. 4,919,912 describes a method in which a SO2-containing gas stream is passed through a reaction zone in which a Claus reaction takes place and elemental sulfur is formed. Since the gas stream contains a superstoichiometric quantity of SO2 for the Claus reaction, the waste gas from this Claus reaction is fed to a hydrogenation zone in which the excess SO2 is converted into hydrogen sulfide. The hydrogen sulfide is extracted from the outflowing gas and fed back to the reaction zone and is available for producing sulfur with the Claus reaction.